It is well-known that the purity of water-soluble dyes are paramount toward the reliability performance of ink-jet inks. Commercially-available dyes often contain significant amounts of organic and inorganic impurities which are undesirable for digital printing applications, such as thermal ink-jet printers. Residual impurities degrade reliability performance by kogating on the resistor surface as well as crusting at nozzles. Some of the impurities found in water-soluble dyes are sodium chloride and sodium sulfate.
Macromolecular chromophores (MMCs) are derived through chemical treatment of pigments, whereby water-solubilizing groups are bonded covalently on the pigment particle. Colorants of this type are water-soluble and self-dispersible without requiring the use of dispersing agents.
Two commercial suppliers of these MMCs are Orient Chemicals and Cabot Corporation. During the chemical treatment, unreacted starting materials and by-products are found to remain on the MMCs. Through chemical analysis, it has been determined that the amounts of impurities exceed those of ink-jet grade dyes. Crusting is particularly evident in commercially-available MMCs.
At least two processes are presently known for modifying ink-jet grade dyes. These include (1) reverse osmosis, as disclosed, for example, in U.S. Pat. No. 4,685,968, issued Aug. 11, 1987, to Donald J. Palmer and assigned to the same assignee as the present application, and (2) ion exchange, as disclosed, for example, in U.S. Pat. No. 4,786,327, issued Nov. 22, 1988, to Donald E. Wenzel et al and assigned to the same assignee as the present application. In both instances, the primary purpose of the respective process is to effect a change of the counter-ion associated with the dye. However, both patents suggest the use of reverse osmosis to purify the dye.
While the foregoing references are certainly useful for purifying dyes used in ink-jet inks, more recent advances have resulted in the use of pigments, particularly pigments modified in some fashion, to produce MMCs. A process is required for purifying MMCs to a purity level acceptable for ink-jet applications.